The principle of laser machining

Laser processing technology (FIG. 1) forms a laser beam with high energy density by focusing the energy of light through the lens, utilize the properties of the laser beam and material interaction for cutting, welding, surface treatment, punching and micro-machining (including metal and non-metal).

Application of laser technology

Laser cutting

Although the cutting speed is fast and the cutting thickness is large, the cutting size precision is obviously not good. The cutting cost is large and the subsequent processing cost is higher.

Although the machining cutting has high accuracy, the cutting speed is too slow and can’t cut the complicated curve. Besides, the material loss is serious during cutting.

The blanking cutting efficiency is higher, the cost is the lowest, but the processing quality is not high, the processing scope is limited. The cutting quality is very poor especially in processing the thick plate and the complex curve shape plate.

Although plasma cutting is more efficient, the cutting section is better than that of the above methods, but the cutting precision can only reach the millimeter level. In other words, it can only be used for rough machining and semi-finish machining.

When machining, the incision is smooth because it replaces the traditional tool or flame with a light beam. There is no need for further processing.

The cutting heat affected area is small.

Small sheet deformation

Small cutting seam (high utilization rate)

There is no mechanical stress in the incision

No shear burr

High machining accuracy

Good repeatability

Do not damage plate surface.

CNC programming

Don’t need to open mold

Economic and time saving

Especially when machining curve, the advantage is most obvious.

Compared with blanking cutting, the processing surface is smooth, and there is no obvious blade trace in the blanking machining curve.

Because the plate is stationary during processing, it also avoids the scratch when the plate surface moves.

Laser cutting is to use the focused high power density laser beam to illuminate the workpiece, so that the material that is irradiated rapidly melts, vaporizes, ablates or reaches the point of ignition.

At the same time, the workpiece is cut off by blowing the molten material with the high-speed airflow with the same axis of the beam.

Laser cutting is one of the hot cutting methods.

Laser cutting can be divided into four categories:

laser vaporization cutting

laser melting cutting

laser oxygen cutting

laser scribing and rupture control

(1) Laser vaporization cutting

Laser vaporization is used to heat the workpiece by the laser beam with high energy density. The temperature rises rapidly and reaches the boiling point of the material in a very short period of time. The material began to vaporize and form steam. The steam spewed out very fast. When the steam blows out, an incision is made on the material.

Laser vaporization is mainly used for cutting of extremely thin metal materials and non-metallic materials.

(2) Laser melting cutting

During laser melting cutting, the metal material is melted by laser heating. Then the non-oxidizing gas (Ar, He, N2, etc.) is sprayed by the nozzle of the same axis with the beam. The liquid metal is ejected by the powerful pressure of the gas, forming an incision.

The laser fusion cutting does not require the complete vaporization of the metal, the required energy is only 1/10 of the vaporization.

Laser fusion cutting is mainly used for the cutting of non-oxidizable materials or active metals, such as stainless steel, titanium, aluminum and alloys.

(3) Laser oxygen cutting

The principle of laser oxygen cutting is similar to that of oxyacetylene cutting.

It uses the laser as the preheating source and uses the oxygen and other active gases as the cutting gas.

On the one hand, the gas produced by the jet has an oxidation reaction and a large amount of oxidation heat. On the other hand, the molten oxide and the melt are blown out of the reaction area, forming an incision in the metal.

As the oxidation reaction in the cutting process produces a lot of heat, the energy required for laser oxygen cutting is only 1/2 of the melting cut. However, the cutting speed is much larger than the laser vaporization cutting and melting cutting.

Laser scribing utilizes laser with high energy density to scan the surface of brittle materials, heat the material into a small groove. Then apply a certain pressure, the brittle material will crack along the slot.

The first three cutting methods are the main application of laser cutting.

From the current situation of the laser cutting machine in the market, the effect of laser on cutting black metal is better. The cutting speed is fast and the cutting thickness can reach 20mm or higher.

Because of the reflection effect of nonferrous metal’s molecular structure to the laser beam, the cutting effect of nonferrous metal is slightly worse. And the machine must be equipped with a reflector.

According to statistics, the thickness of the cut aluminum alloy is not more than half of the black metal.

Cutting copper alloy effect is worse, especially cutting copper.

The core part of laser cutting is a laser generator, include CO2 laser and the fiber laser generator.

CO2 laser generator

The CO2 laser generator is discharging through a mixture of CO2, He and N2 in the cavity of the laser by high pressure.

The atoms in the mixture are stimulated to release energy, and the energy is output in the form of photons or electrons to form a laser.

The laser emitted by the CO2 laser is visible light, which causes slight damage to the retina and skin. Operators are advised to wear protective glasses.

Under the action of pump light, high power density can easily be formed in the optical fiber, which causes the laser energy level of the laser working substance to “reverse the number of particles”. The laser oscillator output can be formed when the positive feedback loop is added in due course.

The output is not visible light, which causes severe damage to the retina and skin. The operator must wear special protective glasses during operation.

The optical path structure of CO2 laser is more complex and optical lens loss is larger. It also has higher environmental requirements (less dust).

The machine requires isolation from the earthquake focus and ensure that the laser is in a dry and constant temperature.

Fiber laser light path is simple with not high environmental requirements (high tolerance for dust, vibrate, shock, temperature and humidity).

The fiber laser is faster when cutting thin plate, and the CO2 laser is stronger when cutting thick plate.

Laser welding

Laser welding (figure 3) is an important field of laser technology.

Fig.3 Laser welding

Laser welding works by using high energy laser pulses to heat the material in small areas. The energy of the laser radiation diffuses through the heat conduction to the material, and the material is melted down to form a specific molten pool.

(1) During laser welding, the amount of heat input can be reduced to the minimum requirement, and the range of metallographic variation in the thermal effect area is small, and the deformation caused by heat conduction is also minimal.

(2) After confirm the single channel welding parameters of 32mm plate thick are qualified, which can reduce the time required for thick plate welding and even eliminate the use of filler metal.

(3) No electrodes are needed, no contamination or damage concerns. And because it does not belong to the contact welding process, the loss and deformation of the fixture can be minimized.

(4) The laser beam can be easily focused, aligned and guided by an optical instrument. It can be placed at an appropriate distance from the workpiece, and can be redirected between the machine tools or obstacles around the workpiece. Other welding laws are limited by the above space constraints.

(5) Workpieces can be placed in enclosed Spaces (controlled by a vacuum or internal gas environment).

(6) The laser beam can be focused on a small area and can be soldered with small and spaced parts.

(7) A wide range of weldable materials and can be used for seaming various heterogeneous materials.

(8) Easy to weld in high-speed and automatically, or controlled by digital or computer.

(9) When welding thin material or thin diameter wire, it will not be as easy as arc welding.

(10) It is not affected by the magnetic field (arc welding and electron beam welding is easy to be affected), can accurately align the welding parts.

(11) Two kinds of metals in different properties can be welded, such as different resistances.

(12) If in perforated welding, the depth weld ratio can reach 10:1.

(13) You can switch the device to transfer the laser beam to the multi-workstation.

Because of the above characteristics of laser welding, laser welding is widely used in the field of civil vehicle manufacturing.

Laser welding is the main welding process in the manufacturing of high-speed railway and automobile.

But laser welding also has many disadvantages, which include:

(1) The position of the welding parts should be very precise and must be within the focus range of the laser beam.

(2) When the welding parts need clamps, must ensure that the final position of the weld is aligned with the welding spot where will be impacted by the laser beam.

(4) The welding properties of high reflectance and high thermal conductivity materials such as aluminum, copper, and alloys will be changed by the laser.

(5) When using high energy laser beam welding, plasma controller is used to removing the ionized gas around the molten pool to ensure the reconstruction of the welding channel.

(6) Energy conversion efficiency is too low, usually less than 10%.

(7) Fast solidification of the weld bead, may have stoma and embrittlement.

(8) Expensive

In particular, the last disadvantage is that the equipment is expensive, greatly restricted the application of laser welding technology.

Laser engraving

Laser engraving works with a laser beam with extremely high energy density under the control of the computer, irradiate the surface of the product which required marking to make the surface of the product melt or vaporize instantly, thus marking the product surface with the required text or logo, as shown in figure 4.

Laser engraving is the most popular advanced marking method globally, which is very suitable for modern production (high efficiency and fast pace).

As shown in table 1, the comparison of several marking techniques. It can be seen from this that the advantages of laser marking technology are very obvious.

Marking Tech

Performance

Effect & Accuracy

Marking Color

Graph Changes

Consumables

Laser marking

Good

High accuracy and good effect

Determined by material

At will

No

Chemical etching

Good

Low accuracy

Material color

Not easy

Yes

Ink printing

Worse

High accuracy

Any colors

Easy

Yes

Mechanical engraving

Better

Low accuracy

Material color

At will

Yes

Mechanical press

Worse

Poor accuracy

Material color

Not easy

Yes

Table 1. The comparison of several marking techniques

Laser rapid prototyping technology

Laser rapid prototyping (FIG. 5) is a major change in modern manufacturing technology. It is an extension of laser technology in industrial application.

Fig.5 Laser rapid prototyping technology

With the acceleration of the global market integration process, the competition of the manufacturing industry is very fierce, and the development speed of products becomes the main contradiction of competition.

At the same time, the manufacturing industry needs to meet the ever-changing needs of the users, and also requires the manufacturing technology to be more flexible, which can be produced in small batches or even single parts without increasing the cost of the product.

RP-Rapid Prototyping technology is a manufacturing method that accumulates the material layer by layer. Or, more generally, a computer designed 3D graphics.

The high temperature produced by the laser is used to sintered metal powder in 3D graphics, producing metal components.

The prototype can be made directly from CAD 3D solid model in a few hours or dozens of hours.

Rapid prototyping provides a richer, more intuitive entity than the information provided by the drawings and computer screens.

Especially in the stage of product development, considering various factors comprehensively and use the rapid prototyping technology to enable the development to be successful at once.

Combined laser rapid prototyping technology with the existing precision casting process in the foundry to make the foundry shop has the ability to quickly produce all kinds of the wax molds used for precision casting in large size and complex structure, which will reduce a lot of outsourcing costs.

At the same time, the production of a single and small batch of precision castings can be used without the mold, thus saving a lot of tooling costs and greatly shortening the production cycle.

The development and development of new products save a lot of precious time and reduces the production cost.

Moreover, the precision casting level of foundry shop has been improved, so as to lay a good foundation for the successful completion of precision casting production tasks in the follow-up products.

It is certain that the future laser rapid prototyping technology will be more widely used.

Laser heat treatment

Laser heat treatment (FIG. 6) is a surface heat treatment technology. That is to use the laser to heat the metal material surface to achieve surface heat treatment.

Fig.6 Laser heat treatment

It can be used for metal surfaces modification treatment such as harden metal (or it is called surface quenching, surface amorphous, surface re-melting and quenching.), surface alloying and other surface modification. A change in surface composition, organization, and performance that is not achieved by the large surface quenching.

After laser treatment, the surface hardness of cast iron can reach 60HRC, and carbon steel with medium carbon and high carbon can reach up to 70HRC.

Thus, it can improve its wear resistance, fatigue resistance, corrosion resistance and oxidation resistance, and prolong its service life.

Conclusion

Because of laser processing technology has many advantages, laser in industrial manufacturing shows that the low cost, high efficiency and application of the huge potential, make the world’s major industrial countries compete with each other.

Laser technology is expanding in new application fields, and the development speed is amazing.

In the main large manufacturing, such as automobiles, electronics, machinery, aviation, steel and other industries, some countries have basically completed the changes from traditional processing technology to laser processing and enter into the era of “light”.

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